The present invention relates to semiconductor circuitry and in particular to improving the high current operation of a semiconductor circuit by implanting impurities at an angle to create a local implant with increased area.
Semiconductor junctions are often limited in their high current region of operation by leakage currents. In the case of bipolar transistors, as the collector current increases, electron and hole injection increases the base depth into the lightly doped collector region, thus reducing the gain of the transistor. Similar effects are observed in metal oxide semiconductor transistors. One method for improving the high current operation of bipolar transistors include increasing the collector doping concentration throughout its junction with the base via a perpendicular implant of appropriate doping through the base opening as shown in prior art FIG. 1. The peak current at which transistor gain starts to drop off due to high current effects increases, but since the entire collector region vertically adjacent the base is more heavily doped, the base-collector capacitance is significantly increased. Ideally, high current operation should be maximized and base-collector capacitance should be minimized for optimum performance. Similar effects are obtained by increasing the doping of the entire collector region. Since speed is of premium importance in such transistors, there is a need for a solution which does not degrade performance at high frequencies.
A second method shown in prior art FIG. 2 reduces the base-collector capacitance of the first method, which improves the high frequency response, by using a local implant through the emitter to increase the collector doping below the emitter opening only, where the carrier injection into the base mostly occurs at high currents. However, since the highest emitter current density occurs at the emitter edges, scattered carriers bypass the implant around its perimeter, again causing transistor gain loss at high currents. There is a need for both improving the high frequency operation and the high current operation of bipolar transistors without adding many additional processing steps.
A local collector implant of impurities is created in a bipolar transistor by angling the implant through an emitter opening. The localized implant covers an area larger than the emitter opening, thereby minimizing carrier injection around the perimeter of the implant at high currents. In addition to improved high current operation, high frequency operation is also improved over previous methods of increasing the entire collector doping concentration where it contacts the base or performing a conventional local collector implant through the emitter opening. Only one additional implant is required over normal bipolar transistor formation processes, and no additional masking is required
In one preferred embodiment, a standard ion beam source using phosphorus as the impurity for a npn transistor is used to perform the angled implant For a pnp transistor, a p source of impurities is used, such as boron. The impurities are accelerated from the ion source, and a mass spectrometer is used to separate undesired impurities. A high energy, fairly narrow ion beam is focussed and scanned across the emitter opening at an angle to a line perpendicular to the surface of the emitter, resulting in a wider implant at the collector-base junction, which injected carriers have a harder time circumventing. The angling of the beam is accomplished by tilting the silicon wafer on which the transistor is being formed with respect to the source of ions.